(a) Field of the Invention
The present invention relates to a micro/nano imprint lithography technique and in particular, to a stamp that is used in an UV-micro/nanoimprint lithography process or thermal micro/nano imprint lithography process and a method for fabricating the stamp.
(b) Description of the Related Art
Micro/nano imprint lithography is a technique to fabricate micro/nano-structures effectively and economically and uses a stamp with embossed micro/nano-structure thereon in a manner that the stamp is pressed on the surface of resist applied on a substrate for imprinting the micro/nano-structure repeatedly.
Professor Chou suggested the previous microimprint lithography technique in which a stamp with embossed structures in microscale is fabricated first by electron-beam lithography, followed by being contacted with the surface of a resist made of polymethylmethacrylate (PMMA) applied on a substrate, and the stamp is pressed in high temperature and separated off from the substrate after cool-down.
In the method aforementioned, a reversed structure to the structure embossed on the stamp is imprinted on the resist of the substrate, and the residual resist in the pressed area is removed by anisotropic etching.
Another imprint lithography technique is Laser-Assisted Direct Imprint (LADI) in which the imprint is carried out by a single 20 ns excimer laser with a wavelength of 308 nm making a resist coated on a silicon wafer (substrate) melted down instantly.
Another imprint lithography technique is Nanosecond laser-assisted nanoimprint lithography (LA-NIL) in which a polymer material is used as a resist, and micro-structures with 100 nm in line-width and 90 nm in depth may be imprinted thereon.
All of the imprint techniques aforementioned use heat to cure the resist.
However, such an imprint technique using heat has difficulty in multiple alignment due to the thermal deformation of the resist when applied for semiconductor fabrication where a multi -layer process is essential. Also, pressure as high as about 30 atms applied to the stamp for imprinting the high viscous resist may cause the break down of the structure made of a heat curable material. Furthermore, the opaqueness of the material used for the stamp substrate causes difficulty in multi-alignment during the process for fabricating the micro-structures.
Step & Flash Imprint Lithography (SFIL), proposed by professor Sreenivasan et al., is a technique to solve the problems of such a thermal imprint technique.
The SFIL technique features an ultraviolet (UV) ray-curable material for the resist that enables the fabrication of micro/nano-structures in room temperature and low pressure. It also features an UV ray-transparent material for the stamp substrate such as quartz and Pyrex glass.
In the SFIL processes, a transfer layer is firstly coated on a silicon substrate. Then UV ray -curable resin with low viscosity flows in so as to fill the micro-structure of the stamp by surface -tension while the UV ray-transparent stamp is kept from the transfer layer with a predetermined gap. At the time the filling completes, the stamp gets contacted with the transfer layer, and UV-ray is applied in order to cure the resin. Then the stamp is separated, and a micro-structure is imprinted finally on the substrate though etching and lift-off processes.
Recent studies are carried out on the way to press by the stamp the UV ray-curable resin drop placed between the transfer layer and the stamp in the SFIL process.
Another feature of the SFIL process is the adoption of multiple imprinting by a small unit -sized stamp instead of one imprinting on the entire substrate in a step and repeat way.
Though this method is efficient to fill the micro-structure due to the small stamping area, it is a shortcoming to take a long time for a substrate with a large base area to be imprinted because each stamping should go through its alignment and imprint.
On the other hand, the stamp for the previous microimprint lithography has an anti-sticking layer formed on the stamping surface thereof to prevent the stamp from being stuck with the resist.
However, in the case that the anti-sticking layer is formed on top of the micro/nano-structure smaller in dimension than 50 nm, the anti-sticking layer may distort the shape of the pattern embossed on the surface by the thickness of the anti-sticking layer so that the unwanted pattern may be obtained. Also, the structural stability of the anti-sticking layer may be deteriorated when the thickness thereof is reduced.